The theoretical energy density of aluminum per unit volume is 8,050 Ah/l, which is equivalent to approximately 4 times that of lithium. Accordingly, if aluminum or an aluminum alloy can be used as the negative electrode of a cell, then a cell having a high energy density can be realized at low cost. Further, because the electrode potential of aluminum is −1.66 V (vs. the standard hydrogen electrode), indicating lower potential, by combining aluminum with an appropriate positive electrode active material, there is a possibility of achieving interchangeability with existing cells, and for these reasons, cells using aluminum or an aluminum alloy as the negative electrode are a viable prospect. For example, an acid-base concentration cell using a normal temperature molten salt (ionic liquid) obtained from 1,3-dialkylimidazolium chloride and aluminum chloride as the electrolyte and having aluminum as an electrode is already known (Non-Patent Document 3). Moreover, if scrap aluminum could be reused as cell electrodes, this would be very desirable from the viewpoint of effective utilization of natural resources.
Examples of known cells which use aluminum as the negative electrode include aluminum-halogen fuel cells. These aluminum-halogen fuel cells are cells which extract the energy generated when the aluminum fuel is halogenated as electrical power. For example, Patent Document 1, Non-Patent Document 1 and Non-Patent Document 2 disclose an aluminum-chlorine fuel cell in which a graphite tubular electrode and a molten aluminum electrode are placed in a molten electrolyte at 750° C., chlorine gas is supplied to the graphite tubular electrode, and the electrical power generated between the graphite tubular electrode and the molten aluminum electrode is extracted. In this fuel cell, a mixture of 75% sodium chloride and 25% magnesium chloride is used as the molten electrolyte. In this cell, if the surface of the molten aluminum electrode becomes coated with an oxide film during power generation, then the resistance increases markedly, and the power generation efficiency deteriorates. Accordingly, Patent Document 1 proposes a system in which argon gas is blown onto the molten aluminum electrode, and an electromagnet is used to apply an alternating magnetic field, thereby inhibiting the production of an oxide film and preventing any deterioration in the power generation efficiency.
On the other hand, aluminum chloride or aluminum bromide is produced, for example, by blowing chlorine gas or bromine gas onto molten aluminum metal, sublimating the produced aluminum chloride or aluminum bromide, and then performing cooling. This production requires a large-scale apparatus and a large amount of energy to melt the aluminum metal.
Patent Documents 2 and 3 propose non-aqueous electrolyte secondary cells using aluminum as the negative electrode, in which a non-aqueous electrolyte containing a non-aqueous solvent such as 1,2-dichlorobenzene, an aluminum halide such as aluminum bromide and an organic halide is used.
Examples of the organic halide include quaternary ammonium salts such as trimethylbenzylammonium chloride, quaternary phosphonium salts such as tetra-n-butylphenylphosphonium chloride, N-alkyl substituted pyrrolidinium salts such as N,N-dimethylpyrrolidinium chloride, N-alkyl substituted pyridinium salts such as N-n-butylpyridinium chloride, and N-alkyl substituted imidazolium salts such as 1-ethyl-3-methylimidazolium chloride.